화학공학소재연구정보센터
Bulletin des Centres de Recherches Exploration-Production Elf Aquitaine, Vol.20, No.1, 61-90, 1996
Eifelian-Givetian siliciclastic carbonate ramp systems .2. Cyclostratigraphy and palaeostructuration
The French-Belgian Eifelian-Givetian (E-G) transitional beds (100-250 metres thick) developed along a storm-dominated pericratonic ramp (PREAT & KASIMI, 1995). The locus of this second parr is on the small-scale cyclicity and its probable control by Milankovitch forced sea-level oscillations. These E-G layers consist of ten types (cycles 1 to 10) of 5(th) order upward-shallowing cycles (1 to 15 metres thick), Cycle durations calculated along the ramp profile are between 17 and 53 ky and about 35 ky on average, The cycles are ;symmetric and can be grouped into three major types: deep subtidal (cycles 1 to 3), shallow subtidal (cycles 4 to 8) and peritidal cycles (cycles 9 and 3) which are genetically related and constitute a continuous series along the ramp profile. Deep subtidal cycles consist of basinal argillites deposited below storm wave base, overlain by storm-deposited line-grained siltstones and shelly limestones; shallow subtidal cycles consist of stacked skeletal packstone/grainstone deposits and reefal floatstone/rudstone biostromes Peritidal cycles are composed oi very shallow subtidal to intertidal semi-restricted algal/calcisphaerid/ostracod fenestral wackestones and laminites. The latter oi these display millimetre-thick laminations formed by algal binding of thin storm layers. The 5(th) order cycles display the same stacking pattern within the six studied profiles end record an overall regressive prograding depositional sequence. These 5(th) order cycles are stacked to form several 4(th) order parasequence sets allowing definition of a regional third-order sequence of type 2 (sensu VAIL et al,, 1991). The overall characteristics oi these 5(th) order cycles i.e., 1) large lateral extension; 2) pronounced asymmetry; 3) average duration between 17 and 53 ky; 4) a 1:5 bundling within the 4(th) order parasequence sets; 5) a simultaneous development of subtidal and peritidal cycles, suggest a Milankovitch type climatic-eustatic control. The 1:5 recurrence ratio of metre-scale cycles within the megacycles could represent the precession signal modulated by the short eccentricity signal. These eustatic sea-level fluctuations which are in accordance with the Milankovitch frequencies during the non-glacial E/G period may be associated with lake and/or underground aquifer volume variations. Systematic changes in the stacking patterns of the 5(th) order cycles are used in conjunction with Fischer plots to define long-term sea-level changes, i.e, to define the 3(rd) order depositional sequence associated to the E/G transitional series, The E/G shelf margin systems tract (SMST) followed a long-term sea-level highstand (HST, cycles 1 to 3) and developed during a rapid sea-level fall (type-2 sequence boundary), followed by a long-term lowstand systems tract (cycles 4 to 10), The absence of any subaerial erosion err karstification indicates that the sea-level fall was not important and did not exceed the subsidence rate. The absence oi emersion features, particularly within the crinoidal and reefal buildups suggests that the amplitude of the sea-level fall was less than 20-25 metres during the E/G transition, Based on conodont zonation, the duration of the lowstand systems tract was approximately 700 ky, so the relative sea-level fall can be estimated to have been between 3 and 3.5 cm/1000 y. This value is in the range of the tectono-eustatic sea-level fluctuations.